MozbBackup. Backup your Firefox bookmarks, extensions, history etc. Free and pretty useful if you are a Firefox power user (although I had to say that it failed to backup my 38 + extensions, and just stalled). Nevertheless definitely worth a look.
MozBackup is a simple utility for creating backups of Mozilla Firefox, Mozilla Thunderbird, SeaMonkey, Mozilla Suite and Netscape profiles. It allows you to backup and restore bookmarks, mail, contacts, history, extensions, cache etc. It’s easy way to do Firefox backup, Thunderbird backup …This program is freeware and works on Windows 98/ME/NT/2000/XP/2003. It’s developed in Czech republic.
check out FEBE — same thing, and it always works for me…
http://customsoftwareconsult.com/extensions/febe/febe.html
Yeah I have done and it works, but the only hassle is that it has no restore function that I can see, and it sticks all your extensions into a folder willy nilly, without parcelling them all up neatly into a zip file or anything. Which is kind of sloppy to my thinking.